1. Field of the Invention
The present invention relates to a color filter substrate for use in a color liquid crystal display (LCD) and other display devices.
2. Description of the Related Art
LCDs are relatively small, thin and lightweight display devices with comparatively low power dissipation. By taking advantage of these features, LCDs are currently used extensively in a broad variety of electronic appliances. Among other things, active-matrix-addressed LCDs with switching elements are used particularly widely in office automation (OA) equipment including personal computers, audiovisual (AV) appliances such as TV sets, and cell phones. Meanwhile, the size, definition, effective pixel area ratio (i.e., aperture ratio), viewing angle, color purity and other quality parameters of LCDs have recently been increased or improved by leaps and bounds.
An LCD for a big size TV monitor, in particular, is recently required to increase its response speed and improve its viewing angle characteristic (or widen its viewing angle range) more than anything else. However, the conventional twisted nematic (TN) or super twisted nematic (STN) mode LCDs have just narrow viewing angle ranges, which is a problem. Thus, to overcome this problem, various technologies have been developed so far.
A multi-domain vertical alignment (MVA) mode LCD is a typical product of those new technologies (see Japanese Patent Application Laid-Open Publication No. 11-242225, for example). The MVA mode LCD includes a protrusion for controlling the pretilt angles of liquid crystal molecules (i.e., controlling the orientations thereof) on the surface of a substrate and an electrode with a slit or any other form of opening. The protrusion and the slit contribute to grouping the orientations of the liquid crystal molecules, thus improving the viewing angle characteristic and increasing the response speed of the liquid crystal molecules. For example, Example 5 of Japanese Patent Application Laid-Open Publication No. 11-242225 discloses an MVA mode LCD in which multiple columns of protrusions are provided for a counter electrode on a color filter substrate and a plurality of slits are cut through pixel electrodes on an active-matrix substrate opposed to the color filter substrate. Japanese Patent Application Laid-Open Publication No. 11-242225 also discloses an arrangement in which both the pixel electrodes and the counter electrode have slits and an arrangement in which both the pixel electrodes and the counter electrode have protrusions.
Meanwhile, to fabricate an LCD panel, a liquid crystal material needs to be injected into the gap between an active-matrix substrate and a color filter substrate facing each other. More specifically, a seal resin is applied onto the inside edges of the active-matrix substrate and the color filter substrate, thereby forming sealing portions. Next, the sealing portions of these substrates are bonded together such that a predetermined gap is left between the two substrates. Thereafter, a liquid crystal material is injected into this gap and then the resultant liquid crystal cell is sealed up. A vacuum injection process has often been used in the prior art as a method for injecting the liquid crystal material. In the vacuum injection process, first, the liquid crystal cell is loaded into a vacuum chamber, thereby evacuating the liquid crystal cell and creating and holding a vacuum in the cell. Next, an injection port, provided in the sealing portions, is put into a liquid crystal material stored in a container. Thereafter, the pressure in the chamber is raised back to the ordinary pressure again, thereby making the liquid crystal material permeate the liquid crystal cell at the atmospheric pressure. Then, the injection port is sealed with a resin. However, according to this vacuum injection process, it takes a lot of time to finish injecting the liquid crystal material. This problem gets even more serious when this process is applied to making a big LCD panel for a big-screen TV.
Thus, to avoid this problem, the injection process has recently been replaced by a dropping process more and more often (see Japanese Patent Gazette for Opposition No. 8-20627, for example). In the dropping process, a sealing portion is provided on the surface of one of two substrates facing each other, a liquid crystal material is dropped onto the area inside the sealing portion, and then the substrate with this sealing portion and the other substrate are bonded together and the sealing portion is cured. According to this dropping process, a liquid crystal cell can be obtained while the liquid crystal material injected is being encapsulated.
In the dropping process, however, it is difficult to constantly drop the same amount of liquid crystal material onto the sealing portion per unit time at a controlled rate. That is why some bubbles may remain in the liquid crystal cell due to the inconstant dropping rate. Those bubbles will be referred to herein as “residual bubbles”. The techniques of eliminating those residual bubbles from the dropping process are disclosed by Japanese Patent Application Laid-Open Publications No. 2002-107740 and No. 11-174477, for example. Specifically, Japanese Patent Application Laid-Open Publication No. 2002-107740 discloses a method for preventing residual bubbles from being produced due to the lack of the liquid crystal material injected by collecting the liquid crystal material once dropped if the amount of the liquid crystal material dropped has deviated from a reference value. On the other hand, Japanese Patent Application Laid-Open Publication No. 11-174477 discloses a method for preventing residual bubbles from being produced due to the emission of gases (e.g., adsorbed water) from color filters in an LCD with a color filter substrate by thermally treating and degassing the color filter substrate and then dropping a liquid crystal material.
Those residual bubbles produced by the dropping process are very noticeable in an MVA mode LCD, in particular. For example, suppose a pixel electrode has a plurality of slits as disclosed in Japanese Patent Application Laid-Open Publication No. 11-242225. In that case, liquid crystal molecules with various different orientations are present around the shorter side of each slit 11 (i.e., the side intersecting with the longer sides thereof) as identified by the reference numerals 12a, 12b, 12c, 12d, 12e and 12f in FIG. 7B. Thus, the liquid crystal molecules have disturbed, abnormal orientations. As a result, not just the viewing angle characteristic deteriorates but also the luminance and response speed of the panel decrease as well. Furthermore, when black display is switched into white display, the white presented on the screen instantaneously becomes brighter than the intended one to produce an after image, too. It should be noted that electrode portions near the shorter sides (i.e., ends) of the slit connect together parts of an electrode that are divided by the slit, and will be referred to herein as “electrically connecting portions” after Japanese Patent Application Laid-Open Publication No. 11-242225.
Thus, in Example 6 of Japanese Patent Application Laid-Open Publication No. 11-242225, multiple columns of protrusions are arranged in a zigzag pattern on the counter electrode on the color filter substrate, while a plurality of slits are arranged in the same zigzag pattern on the pixel electrodes on the active-matrix substrate. In such an arrangement, a black matrix (BM) 140 having openings with a closed cross section is provided as shown in FIG. 7B in order to cover those portions with the abnormal orientations. Japanese Patent Application Laid-Open Publication No. 11-242225 also discloses that the electrically connecting portions 12e and 12f are shielded from light with a storage capacitor electrode, which is provided for the active-matrix substrate to stabilize the potential at the pixel electrodes.
However, if a LCD panel is fabricated by the dropping process using such a BM, then the BM constitutes a barrier, which prevents the liquid crystal material dropped from being distributed sufficiently broadly, thus slowing down the progress of the liquid crystal material and producing bubbles eventually.
A similar problem is observed in any liquid crystal display device including a BM having openings with such a closed cross section, no matter whether the LCD operates in the MVA mode or in the TN mode. Particularly, if the thickness of the liquid crystal layer is reduced to increase the response speed, then the progress of the liquid crystal material will be even slower and the bubbles will be produced even more easily. Also, comparing the MVA mode LCD and the TN mode LCD to each other, the bubbles produced are more noticeable in the MVA mode LCD than in the TN mode LCD. This is because the liquid crystal material used for the MVA mode LCD has higher viscosity than that used for the TN mode LCD. Thus, the liquid crystal material has higher flow resistance around the surface of the alignment film. In addition, the MVA mode LCD has protrusions for controlling the orientations of liquid crystal molecules.
It should be noted that neither the technique disclosed in Japanese Patent Application Laid-Open Publication No. 2002-107740 nor that disclosed in Japanese Patent Application Laid-Open Publication No. 11-174477 was developed to prevent those residual bubbles from being produced by a BM with such openings. Thus, it is difficult to overcome those problems by any of these techniques.
In order to overcome the problems described above, preferred embodiments of the present invention provide a color filter substrate that can eliminate those residual bubbles, which would otherwise be produced when an LCD panel is made by a dropping process, and a display device including such a color filter substrate.